This invention relates to an improved variable frequency oscillator. More specifically the present invention relates to a variable frequency oscillator in the high frequency spectrum having continuously variable incremental fine tuning and a presettable digital display of output frequency.
Variable frequency oscillators are well known in radio electronics. Such oscillators have typically been constructed of an amplifier element and a resonant circuit element with a positive feedback path provided from an amplifier output to input thereby inducing the amplifier to oscillate at the resonant frequency of the tank circuit. In radio communication applications, the frequency of such oscillators was typically set by mechanical adjustment of a tuning capacitor or inductor. More recently, variable frequency oscillators have been tuned to a predesired frequency by varying the voltage applied to a varactor diode functioning as a voltage controlled capacitor in the tank circuit of the oscillator.
In many cases, the frequency of the variable frequency oscillator has been indicated by graduations on a scale mechanically linked to the tuning element whether capacitor or inductor. In other instances a digital frequency counter has been connected to the variable frequency oscillator or at other points of the circuitry of the communications equipment, to provide a digital indication of operating frequency. Typically, a digital frequency counter was connected as an accessory to the communications equipment having a variable frequency oscillator as a part thereof, and the frequency calibrations of the communications equipment were duplicated, albeit generally more accurately, by the visual digital indications of the frequency counter.
A virtually universal practice in radio communication equipment has been to employ heterodyne frequency conversion in reception of radio signals, and in the generation of transmitting signals as well. Thus, except in the instances of simple amplitude modulated (AM) and continuous wave (CW) transmitters, variable frequency oscillators used therein operate at frequencies other than the output frequency of the transmitter. Moreover, it has become a recognized precept that greater isolation and resultant greater frequency stability accrue when a variable frequency oscillator is operated at a frequency other than output frequency of transmitting devices.
Heretofore, there has been no variable frequency oscillator available which combines the advantages of incremental fine tuning with a tuning range throughout the high frequency spectrum and with a digital frequency display having a preprogrammed offsetting count to provide a direct display of output frequency which is different from but related to actual operating frequency of the VFO.